Method for reducing NOX in combustion flue gas using metal-containing additives

Abstract

Various methods for decreasing the amount of nitrogen oxides released to the atmosphere as a component of combustion gas mixtures are provided. The methods specifically provide for the removal of nitric oxide and nitrogen dioxide (NOx) from gas mixtures emitted from stationary combustion systems. In particular, methods for improving efficiency of nitrogen oxide reduction from combustion systems include injecting metal-containing compounds into the main combustion zone and/or the reburning zone of a combustion system. The metal containing compounds react with active combustion species, and these reactions change radical concentrations and significantly improve NOx conversion to molecular nitrogen. The metal-containing additives can be injected with the main fuel, in the main combustion zone, with secondary or reburning fuel addition, or at several locations in the main combustion zone and reburning zone. Optionally, nitrogenous reducing agents and/or overfire air can be injected downstream to further increase NOx reduction.

Description

1. The Field of the InventionThe present invention is directed to processes for reducing nitrogen oxide emissions in combustion systems. More specifically, the present invention provides methods of decreasing the concentration of nitrogen oxides in flue gases emitted to the atmosphere from stationary combustion systems such as boilers, furnaces and incinerators.2. The Relevant TechnologyNitrogen oxides are the major air pollutants emitted by boilers, furnaces, engines, incinerators, and other combustion sources. Nitrogen oxides include nitric oxide (NO), nitrogen dioxide (NO.sub.2), and nitrous oxide (N.sub.2 O). Total NO+NO.sub.2 concentration is usually referred to as NO.sub.x. Combustion sources produce nitrogen oxides mainly in the form of NO. Some NO.sub.2 and N.sub.2 O are also formed, but their concentrations are typically less than 5% of the NO concentration, which is generally in the range of about 200-1000 ppm. Nitrogen oxides are the subject of growing concern because the...

AI Technical Summary

Benefits of technology

The amount of additives in the combustion zone can be in the range of about 1 ppm to about 10,000 ppm in the combustion flue gas, and is preferably in the range of about 10 ppm to about 2000 ppm. The amounts in units of ppm are based on the molar concentration of the metal-containing additive. Solid particles of the additives can be removed from flue gas downstream by particulate control devices. High concentrations of additives are not preferred, since consumption of large amounts of the additives is economically disadvantageous, and additionally increases the load on particulate control devices. On the other hand, the amount of additives should be high enough to enhance NO.sub.x reduction. One of skill in the art can readily determine the optimal

Problems solved by technology

Nitrogen oxides are the subject of growing concern because they are toxic compounds, and are precursors to acid rain and photochemical smog.

However, their capital costs are low and, since no reagents are required, their operating costs are near zero.

Although SCR can relatively easily achieve 80% NO.sub.x reduction, it is far from an ideal solution for NO.sub.x control.

The size of the catalyst bed required to achieve effective NOx reduction is quite large, and use of this large catalyst, with its related installation and system modification requirements, is expensive to implement.

In addition, catalyst deactivation, due to a number of mechanisms, typically limits catalyst life to about four years for coal-fired applications.

The spent catalysts are toxic and pose disposal problems.

Under ideal laboratory con

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